1. Field of the Invention
The present application is related to information handling systems, and more specifically, to information handling systems having variable speed cooling fans.
2. Description of the Related Art
Information handling systems play a vital role in modern society. An information handling system generally processes, compiles, stores, and/or communicates information or data for business, personal or other purposes, thereby allowing users to take advantage of the value of the information. Because technology and information handling needs and requirements vary between different users or applications, information handling systems may also vary regarding what information is handled, how the information is handled, how much information is processed, stored, or communicated, and how quickly and efficiently the information may be processed, stored, or communicated.
The variations in information handling systems allow for these systems to be general or configured for a specific user or specific use such as financial transaction processing, airline reservations, enterprise data storage, or global communications. In addition, information-handling systems may include a variety of hardware and software components that may be configured to process, store, and communicate information and may include one or more computer systems, data storage systems, and networking systems.
A computer system, which is one common type of information handling system, may be designed to give independent computing power to one or a plurality of users. Computer systems may be found in many forms including, for example, mainframes, minicomputers, workstations, servers, clients, personal computers, Internet terminals, notebooks, personal digital assistants, and embedded systems.
The computer system may include microprocessors that require active cooling to operate in a thermal environment recommended by its manufacturer. To achieve adequate cooling, various thermal solutions are used with fans being integral parts of such solutions. Ideally, maximum airflow (fan fully on) allows for best cooling results, however, acoustical noise from the fan and fan power consumption are also at maximum. Therefore, it is desirable to have the ability to gradually vary fan speed in a thermal solution. However, robust implementation of a variable speed fan controller is hard to achieve due to different fan manufacturers"" requirements for supply voltage stability and available signal quality of fan tachometer (rotational speed) output (especially at low rotational speeds).
The noise and power usage problems have been addressed by using pulse width modulated (PWM) signals to periodically interrupt supply voltage to the fan. This provided desirable results regarding fan noise and power consumption, but in some fans it caused premature failures and corruption of logic generating the tachometer output signals from the fan.
Another way that the noise and excess power problems have been addressed was to use a fully integrated fan controller integrated circuit (IC) having an SMB or I2C compatible interface. This allowed for seamless fan control, however, if the fan controller IC implemented direct PWM control, then the same fan reliability issues were still present. A significant drawback of using the fan controller IC was cost.
Linear regulators have been used with power management controllers by setting the fan voltage based on binary states of one or more control signals (essentially, a crude DAC is used as a reference for linear regulator). However, a drawback was requiring dedicated binary outputs and only course resolution for fan speed control (e.g., most often there are only three fan speed settings: OFF, LOW, HIGH).
Heretofore, low fan speeds could not be utilized below a certain voltage because the fan digital circuits for producing a rotational speed tachometer output were failing to produce acceptable logical levels due to the low supply voltage to the fan motor. In other words, the supply voltage would be sufficient to run the fan at a low speed but the tachometer output became non-functional at low speeds corresponding to the low voltages.
FIG. 1 illustrates relevant components of an information handling system 10 having a central processing unit (CPU) 12 coupled to a memory 14 that stores instructions executable by the CPU 12. Information handling system 10 includes an electric fan motor 16 that turns a fan blade (not shown) for cooling the CPU 12 during operation thereof. CPUs 12 require active cooling to operate in a thermal environmental envelope recommended by the processor manufacturer. Fans are the preferred means for maintaining CPU temperature within the recommended thermal envelope. Ideally, the maximum airflow (fan is fully on) provides the best cooling results. However, it is desirable to be able to gradually vary the fan speed according to the cooling needs in order to save power. Additionally, reducing fan speed reduces acoustic noise produced by the cooling fan. The fan speed can be varied by varying the voltage provided to the power input node of the electric fan motor 16.
Fan speed depends on the magnitude of voltage provided to the fan motor 16. Information handling system 10 includes a circuit for regulating the power provided to the fan motor 16. The circuit includes a power management circuit (PMC) 18 and power field effect transistor (FET) 20 coupled between the electric motor 16 and PMC 18. More particularly, the output of the PMC 18 is coupled to a gate-input node of the FET 20. The source node of the FET 20 is coupled to a first power supply having a voltage VCC1, while a drain node of the FET 20 is coupled to a power input node of the fan motor 16.
The PMC 18 generates a square wave signal, the duty cycle of which depends upon a control signal provided to the PMC 18. FIG. 2 illustrates an exemplary square wave generated by the PMC 18. The square wave shown in FIG. 2 varies between VCC2, the voltage of a second power supply coupled to the PMC 18 in FIG. 1, and ground. VCC2 may be distinct from VCC1, or from the same power supply. The first power supply is capable of providing high current power to the fan motor 16 when compared to the current that is provided by the second power supply. As noted above, the duty cycle depends upon the control signal provided to the PMC 18. The period of the square wave shown in FIG. 2 remains constant, notwithstanding a change in the duty cycle in response to a change in the control signal provided to the PMC 18.
The square wave signal generated by the PMC 18 is coupled to the gate-input node of the power FET 20. When the voltage of the square wave signal is at VCC1, the FET 20 activates (becomes a low resistance), thereby coupling the first power supply (VCC1) to the power-input node of fan motor 16. In response, a shaft (not shown) of motor 16 rotates thereby turning a fan blade (not shown) which in turn produces airflow over the CPU 12. When the voltage of the square wave signal provided to the input gate of FET 20 is at or near ground, the FET 20 turns off, thereby disconnecting the first power supply from the input node of the fan motor 16. In response, the rotational speed of the motor shaft begins to slow and may even stop until the FET 20 is again activated by the square wave being at VCC1.
The rotational speed of the fan motor""s shaft depends upon the duty cycle of the square wave provided to the gate input of the FET 20. The higher the duty cycle results in the higher the average rotational speed of the shaft. To obtain the highest average rotational speed, the duty cycle of the square wave would be 100%. With a 0% duty cycle, no power is provided to the fan motor 16, and the shaft does not rotate. For duty cycles between 0 and 100%, the average rotational speed of the fan motor""s shaft varies accordingly.
The constant coupling and decoupling of the first power supply to the power input node of fan motor 16, according to the square wave provided to the gate input node of FET 20, stresses the fan motor 16 such that it may eventually and prematurely fail. Additionally, the constant coupling and decoupling of the first power supply to the fan motor 16 creates noise which tends to corrupts logic within the fan motor 16 that generates a tachometer signal representing the rotational speed of the shaft of fan motor 16. This tachometer signal is used in controlling the fan motor 16 at a desired rotational speed. Inaccurate tachometer signals will create undesirable and unreliable control of the fan motor 16 when running at low speeds.
Therefore, a problem exists and a solution is desired for improved fan reliability and control in an information handling system.
The present invention remedies the shortcomings of the prior art by providing an apparatus, system, and method for improving fan reliability and control in an information handling system.
The present invention includes benefits of pulse width modulation (PWM) and linear voltage regulation techniques while still keeping the cost down since no dedicated fan controller integrated circuit (IC) is required and the fan controller may be implemented with low cost components, discrete and/or integrated. The variable speed fan controller can drive fans to much lower speeds than other known fan controllers by signal conditioning the tachometer output, according to the present invention.
In an exemplary embodiment of the invention, an existing power management controller (PMC) may be used to generate a PWM output to control fan speed based upon the tachometer output signal from the fan. The PWM control signal is fed into a first order resistor-capacitor (RC) low pass filter that is adapted to output DC voltages proportional to the duty cycle of PWM signal input, e.g., Vout=D*Vcc, where D is the duty cycle [0-1] and Vcc is the supply voltage for the power management controller. The DC output of the low-pass filter may be scaled to match the maximum output voltage level of the PMC (100% duty cycle) and maximum supply voltage allowed to the fan (maximum speed). This scaled voltage is then used as a control signal to a linear voltage regulator, e.g., a series pass transistor, to supply output voltage at high current to the fan that is proportional to the duty cycle of PMC output. Thus, the supply voltage to the fan may be gradually varied from 0 to the maximum power supply voltage value with a single wire interface from the PMC and low cost small sized components.
When the fan supply voltage is brought to a value necessary to obtain the minimum possible fan speed without allowing it to stall, in some fans the tachometer output signal quality will significantly deteriorate and the tachometer output signal will not be recognizable by conventional PMC systems. Under low supply voltage and rotational speed conditions, the signal amplitude on the fan tachometer output may be as low as 400-500 mV and have a gradually increasing DC offset. In the preferred exemplary embodiment of the invention, signal conditioning circuits remove the DC component and amplify this tachometer output signal so that it may be easily recognized by conventional digital logic in the PMC.
In an information handling system, an embodiment of the present invention is characterized as comprising: a microprocessor for executing instructions stored in a memory coupled thereto; a fan for cooling said microprocessor during operation thereof, said fan comprising a fan motor and a fan blade coupled to a shaft of said fan motor, wherein the shaft rotation speed is dependent upon a direct current (DC) voltage level applied to said fan motor, and said fan motor has a tachometer output for indicating the shaft rotation speed; a linear voltage regulator coupled between said fan motor and a first DC power source, said linear voltage regulator controlling the DC voltage level applied to said fan motor; a power management controller having a speed control pulse output and a tachometer pulse input; a pulse-to-DC voltage converter, said pulse-to-DC voltage converter coupled between the speed control pulse output of said power management controller and a control input of said linear voltage regulator; and a tachometer pulse amplifier coupled between the tachometer output of said fan motor and the tachometer pulse input of said power management controller, whereby said power management controller controls said fan motor shaft speed.
Another embodiment of the present invention is characterized as an apparatus for controlling fan speed, comprising: a fan motor having a shaft speed dependent upon a direct current (DC) voltage level applied to said fan motor and a tachometer output; a linear voltage regulator coupled between said fan motor and a first DC power source, said linear voltage regulator controlling the DC voltage level applied to said fan motor; a power management controller having a speed control pulse output and a tachometer pulse input; a pulse-to-DC voltage converter, said pulse-to-DC voltage converter coupled between the speed control pulse output of said power management controller and a control input of said linear voltage regulator; and a tachometer pulse amplifier coupled between the tachometer output of said fan motor and the tachometer pulse input of said power management controller, whereby said power management controller controls said fan motor shaft speed.
Yet another embodiment of the present invention is characterized as a method for controlling a fan motor having a shaft rotation speed dependent upon a direct current (DC) voltage level applied to said fan motor and a tachometer output, said method comprising the steps of: generating a plurality of speed determining pulses, wherein a characteristic of the plurality of speed determining pulses represents a desired fan motor shaft rotation speed; converting the plurality of speed determining pulses into a DC voltage representative of the desired fan motor shaft rotation speed; applying the DC voltage to the fan motor; and determining the shaft rotation speed of the fan motor by measuring a plurality of shaft rotation pulses from the fan motor.
A technical advantage of the present invention is a low cost solution using an existing PMC. Another technical advantage is the ability to interface with most types of fans. Still another technical advantage is gradual and continuous variation of fan speed (PWM to linear conversion). Another technical advantage is a wide range of fan speeds available because of tachometer signal conditioning. Another technical advantage is the ability to vary fan speeds gradually in an analog manner to minimize acoustic noise and save power. Another advantage is reduced cost for parts and labor to implement embodiments of the invention into information handling systems. Still other advantages are support for fans from different vendors and flexibility in using an existing PMC in the information handling system without the need for a special integrated circuit fan controller.